The high penetration of power electronics in the active distribution network (ADN) induces system-level voltage quality issues. Conventionally, pure and hybrid active power filters (APF/HAPF) have been adopted in ADN for voltage quality improvement. However, each APF/HAPF can only compensate for local voltage quality issues due to a lack of coordination. This paper proposes a collaborative distributed optimal control (CoDOC) of APFs/HAPFs for ADN system-level voltage quality improvement. First, the operational characteristic of APF/HAPF is discussed, which formulates the APF/HAPF constraints. Then, the objective functions, harmonic power flow equations, and APF/HAPF operational constraints are proposed to formulate the proposed CoDOC as a convex problem. Moreover, a primal-dual subgradient (PDS) based optimization algorithm is proposed to solve the CoDOC problem in a distributed manner. The practical application aspects of the proposed CoDOC are also studied and discussed. Finally, the proposed CoDOC is verified by case studies compared to other representative ADN-level control methods.